Wessels was a youthteam player at Bayern Munich, who progressed through the reserve team and broke into the first-team squad in September1999 after Bayern lost both goalkeepers Oliver Kahn and Bernd Dreher to injury. Wessels' first game was a 1–1 draw at Ibrox against Rangers in the Champions League. He performed impressively whenever called upon, but was unable to dislodge Kahn for more playing time. Wessels was involved in a hugely successful period for Bayern, including the 2001 Champions League win, but eventually left for Köln in 2003, seeking first-team football. Between 1999 and 2003, he appeared in a total of eighteen matches for Bayern.
Köln
Wessels' time in Köln was eventful, with the club relegated in his first season, before immediately gaining promotion back to the 1st division and again being relegated to the 2. Bundesliga the following season. Showing consistently good performances, Wessels was one of the few players to stand out in the squad. However, at the beginning of the 2007–08 season, Colombian goalkeeper Faryd Mondragón signed with the club, and after a fair but intense rivalry, Wessels lost his position as the club's number 1.
On 6 August 2007, Wessels agreed to a trial in England with Premier League team Everton and subsequently signed an initial one-year contract with the club on 21 August; he was given his favoured number 33 shirt. Upon signing he said: "I am excited to be here in England. It is the first time I have played for a club outside of Germany but the Premier League is a very exciting league and I am looking forward to my time here at Everton." Wessels was the first German player to sign for Everton in the club's history. Following a hand-injury sustained to first choice goalkeeper Tim Howard, Wessels made his Everton debut in the 1–0 home defeat to Manchester United on 15 September 2007. He made his European debut for Everton during a 1–1 draw with Ukrainian team Metalist Kharkiv and he also played against Aston Villa and Sheffield Wednesday, keeping Everton's first clean sheet of the season against the latter. However, he lost his place in the team to the returning Tim Howard for the following game against Middlesbrough. After the game on Sunday 11 May, Everton manager David Moyes revealed that Wessels and fellow team member Thomas Gravesen would leave the club after their contracts expired. David Moyes said "I would like to thank Stefan Wessels who, after today, will be leaving us. His level of professionalism has been a credit to him and he has been a great man to have around the club."
Osnabrück
In June 2008, Wessels signed a two-year contract with VfL Osnabrück of the 2nd Bundesliga. He started the 2008–09 season as their first-choice goalkeeper, making his competitive debut on 8 August 2008 in their 2–0 defeat at FSV Frankfurt in the first round of the DFB-Pokal and going on to make his league debut in a 2–2 draw at FC St. Pauli on 15 August 2008. He played 21 league games for VfL, but left in June 2009, after their relegation from the 2. Bundesliga.
Basel
On 23 October 2009, Wessels signed for the Swiss side FC Basel until the end of the 2009–10 season, as a replacement for injured goalkeeper Franco Costanzo. He will be coached by Thorsten Fink who he played alongside at Bayern Munich. He said "I am very pleased that it has worked out with FC Basel, and the wait has been worthwhile. I'm happy that I've signed with such a big club." He made his debut on 20 November, in a 4–2 Swiss Cup win over rivals FC Zürich at St. Jakob-Park. He then made his league debut nine days later in a 2–0 defeat to BSC Young Boys at the Stade de Suisse, Wankdorf. In July 2010, Wessels joined West Ham United on trial, and played in the 24 Julyfriendly match against Burton Albion, a 4–2 defeat.
Wessels has represented Germany, at various youth levels, up to the under-21 team, for whom he made one appearance. He also played for Team 2006, a Germany B team assembled in preparation for the 2006 World Cup, but has not made it into the senior squad.
